3d Printing Problems
The usual culprits for this sort of thing are as follows:


Too small (scale) : most 3d slicing software has their units set up so that blender should export .stl's at .1 scale.  Otherwise, the object may be waaaay to big. or waaay too small. Try playing with the export scale and the object scale and see if you have any luck.


Too thin : the parallel problem to the scale issue is that if it is too thin, the object will not slice properly.  There is a minimum thickness the printer is capable of, and the slicer will exclude things that are too thin.  
Inverted normals : if the normals are not set the right way, the slicer will not be able to detect the solidity of the object.
Non-Manifold :  Your object must be closed, or the slicer will not be able to detect the solidity of the object.



Due to your description, it sounds like one of the last three.